Many depigmenting agents are known in the prior art. Proteasome is an intracellular multi-enzyme proteolytic complex that is very important in cell maintenance since it is in charge especially of removing damaged proteins (Friguet B. et al., Protein degradation by the proteasome and its implication in ageing, Ann. NY Acad. Sci. (2000) 908: 143-154). The proteasomal system is formed from a catalytic complex, the proteasome 20S and several regulators that influence its activity and its specificity. Association of the regulator 19S with the proteasome 20S forms the proteasome 26S, which performs the degradation of ubiquitin proteins. The proteasome is located in mammalian cells both in the cytosol and the nucleus, and interactions exist with the endoplasmic reticulum and the cell membrane. The proteasome 20S has a molecular mass of 700 kDa and is composed of 14 different subunits encoded by genes either of a type or of β type. The 14 subunits are arranged as a cylindrical stack of four rings of seven subunits, the apical rings being formed from α subunits and the central rings from 13 subunits. This proteolytic complex preferentially cleaves the proteins at the C-terminal end of basic residues (“trypsin-like” activity), hydrophobic residues (“chymotrypsin-like” activity) and acidic residues (“peptidylglutamyl-peptide hydrolase” activity). These peptidase activities are borne by three different β subunits and are located within the structure, thus avoiding the untimely degradation of cell proteins, but posing the problem of accessibility of the active sites to their potential substrates. Finally, during the course of cell ageing, an accumulation of damaged proteins bearing carbonyl groups takes place, which is the signature of modifications of the amino acids by oxidation, which is at least partly explained by a reduction in proteasome activity (Petropoulos, I. et al., Increase of oxidatively modified protein is associated with a decrease of proteasome activity and content in aging epidermal cells. J. Gerontol. A. Biol. Sci. (2000) 55A: B220-227 and Friguet B., Oxidized protein degradation and repair in ageing and oxidative stress, FEBS Letters (2006) 580: 2910-2916).
Moreover, Ando H. et al., in Fatty acids regulate pigmentation via proteasomal degradation of tyrosinase: a new aspect of ubiquitin-proteasome function. J. Biol. Chem. (2004). 279: 15427-33, have demonstrated that, in B16F10 cells (murine melanocyte line that stably expresses and produces melanin), tyrosinase is degraded via proteasome-dependent proteolysis and that this degradation may be stimulated after treatment with linoleic acid or, on the contrary, reduced via treatment with palmitic acid.
The alga Phaeodactylum tricornutum is a diatomaceous unicellular alga that forms part of phytoplankton and that originates from temperate climes.
International patent application WO 02/080 876 in the name of the Applicant discloses the use of an extract of this alga as a cosmetic agent for actively protecting the skin against the harmful effects of exposure to UV or for preventing or retarding the effects of ageing of the skin.
According to said international patent application, the properties of this cosmetic agent are explained by the fact that this extract promotes activation of the proteasome of skin cells, in particular of keratinocytes, thus leading toward promoting the degradation of the oxidized proteins.
A process for preparing a clarified culture medium of at least one photosynthetic marine and/or freshwater microorganism and the use of this clarified culture medium especially in the field of cosmetics has also been described in international patent application WO 2006/008 401.
Although said document mentions, among the possible applications of the clarified culture media, applications as either pigmenting agent or depigmenting agent, it in no way concerns the use of the biomass in itself or of its extracts. What is more, it indicates that the clarified matter of the alga Phaedodactylum tricornutum does not have any depigmenting properties.
It is recalled that the mechanism of formation of skin pigmentation involves the synthesis of melanin in the melanocytes. This mechanism schematically involves the following main steps:Tyrosine→Dopa→Dopaquinone→Dopachrome→Melanin
Tyrosinase is an enzyme that plays an essential role in this sequence of reactions. The tyrosinase especially catalyzes the reaction for the conversion of tyrosine into dopa (dihydroxyphenylalanine) and the reaction for the conversion of dopa into dopaquinone leading to the formation of melanin pigments.
A substance is acknowledged as being a depigmenting agent if it acts directly on melanocytes by inhibiting the activity of these cells or if it blocks one of the steps of melanin biosynthesis. This is the case especially when the substance under consideration inhibits one of the enzymes involved in melanogenesis.